A Spectrum is the Output of the Cosmic Origins Spectroscope's Analysis of Ultraviolet Light: One of the sceince problems to which COS can be applied is the study of gas in the haloes of galaxies. The telescope points at a distant quasar which lies beyond the galaxy shown in the middle image. The spectru of the quasar then shows the gas absorption lines from the galaxy halo.
What UV really does best compared to the optical or infrared or X-ray is spectroscopy. There is a straightforward explanation for this advantage of UV astronomy. The reason is that the primary transitions of almost all the fundamental atoms in the Universe (hydrogen, carbon, oxygen, nitrogen) lie in the ultraviolet, arising from what is known as the ground state.
Atoms interact with light in the visible part of the spectrum, but for the most part the transitions are from excited levels. In the atmospheres of stars, for instance, the densities are high and these upper levels are populated by collisions with other atoms. For this reason, stellar spectra have a rich selection of lines that can be observed from the ground. In interstellar (or intergalactic) space, atoms essentially exist only in their ground states because collisions with other atoms are very infrequent and the upper levels are not populated. In general, ground-state transitions involve ultraviolet photons. We must use UV spectroscopy to study elements through their absorption lines because, except for a very few elements (e.g., sodium (Na), calcium (Ca), potassium (K), and ionized Ca) all of the atomic transitions out of the ground state are in the UV. An important exception to the exclusivity of the UV for studying diffuse matter in space is the emission of light from atoms that glow in the visible because they are excited by UV photons or are recombining from higher ionization states. Planetary nebulae and ionized hydrogen regions around hot stars are good examples that show emission lines in the visible.
That’s why the atmosphere of Earth is opaque, as noted earlier. Ultraviolet light interacts so strongly with atoms that the light cannot get from space to the ground. It also means that once you get into space you can see all those atoms and really understand the physics going on, whereas when you look at them in the optical and/or the infrared, these are the kinds of photons that don’t interact strongly with atoms, that’s how they can make it through the atmosphere. So you’re really using proxies for the true physics, whereas in the ultraviolet you’re getting right at the heart of the atomic physics. If you’re trying to do atomic physics, then most of the Universe is in the ultraviolet. That’s why people really love ultraviolet spectroscopy, but also why it’s difficult. As far as molecular physics goes, the key molecule seen in the UV is molecular hydrogen, generally considered to be the basis for all interstellar molecular chemistry.
Spectroscopy is the real champion, and that’s why there’s been a long history of ultraviolet spectrographs put into space following the early successes with rockets and balloons but not many ultraviolet imagers. Examples of ultraviolet spectrographs in space include those on Copernicus, IUE (International Ultraviolet Explorer), FUSE (Far Ultraviolet Spectroscopic Explorer), EUVE (Extreme Ultraviolet Explorer), as well as the IMAPS (Interstellar Medium Absorption Profile Spectrograph) and ORFEUS (Orbiting Retrievable Far and Extreme Ultraviolet Spectrometer), which were part of the ORFEUS-SPAS space shuttle missions. Major ultraviolet imagers such as GALEX, the Galaxy Evolution Explorer, and Hubble, were late-comers to the game. While you get a totally different view of the Universe in the ultraviolet than you do in the optical, you can use optical images to decide where to point an ultraviolet telescope. On the other hand, infrared and X-ray sources don’t always show up in the optical region, so early infrared and X-ray imaging surveys were essential to the progress of infrared and X-ray astronomy.
GALEX Captures a Shooting Star: Even though Ultraviolet imaging surveys were not critical to the progress of UV spectroscopy for 35 years, once GALEX was launched in 2003 it made major discoveries, enabling new views of the Universe. The Galaxy Evolution Explorer (GALEX) shows a speeding star that is leaving an enormous trail of material that will be recycled into new stars, planets and possibly even life as it hurls through our Galaxy. Mira appears as a small white dot in the bulb-shaped structure at right, and is moving from left to right in this view. The shed material can be seen in light blue. The dots in the picture are stars and distant galaxies. The large blue dot at left is a star that is closer to us than Mira. When astronomers first saw the picture, they were shocked because Mira has been studied for over 400 years yet nothing like this has ever been documented before.